From my previous post Hello To Koreans, I have taught you how to introduce yourself. Now I'm going to explain the grammar part in this post. As I mention in my previous post, Koreans emphasize a lot on respecting and politeness in their culture, so there exist the formal and informal way to say a sentence.

Now, there are two ways you can say "I" in Korean: 저 and 나. We use 저 when we are speaking to strangers, in formal situation or to elder person while 나 is used when talking to a person like our friend or someone younger than us. 저 and 나 indicates the use of formal (존대말) and informal (반말) way so choose the right one depend on the situation.

For "You" in Korean is 너 and 당신. Although using "You" is very common in English, this is different for Korean language. There is no formal way to say "You" because usually in Korea, Koreans will address each other either the person's name or their proper title. 너 refer as "You" and is use for informal way. Now 당신 is a bit special, we don't use 당신 in normal occasion because 당신 refer to the person you are talking to when you have very strong feeling to him/her. So its either you hate him/her very much or you are madly in love with him/her.

For example:

1. 저는 바빠요 - "I'm busy". Always remember to use 저 when talk to stranger to be polite.
2. 브루스씨 안녕하세요 - "Hello Bruce" in English. Notice we don't use 너 or 당신 because that is very inappropriate.
3. 사장님 안녕하세요 - 사장님 refer to boss or president of company. We can address that person with their proper title for example 선생님 if he/she is a teacher or doctor.
4. 당신 사랑해요 - Use 당신 carefully, if you simply refer Koreans as 당신 he/she will be confuse and thought you might wan hit them with something :D

Now back to our title, I'm going to explain the Topic Marker (는,은). Very simple, just add 는 if the word before didn't have any final consonant; add 은 if the word before has a final consonant, and as the name suggested topic marker is place after the topic of the sentence. Take note that topic and subject is different, but topic and subject of a sentence may be the same.

For example:

1. 저는 먹었어요 - "I ate". Notice because 저 didn't have a final consonant, therefore 는 is add after 저.
2. 서현은 진짜 예뻐요 - "SeoHyeon is really pretty". 현 has a final consonant thus 은 is add after 서현 and is pronounce as 서혀는 because the final consonant ㄴ move to fill in the ㅇ. See How to read Korean word for detail.
3. 바빠요 - "I'm busy". Although there is no subject in the sentence, it is understood that the subject is the speaker himself. Koreans will mostly omit the subject in their sentence if it is known who or what is the subject is. This is not common in English so we have to pay attention to what Koreans are talking about to understand who or what they are refer to.

The END~

P/S: I know this look very hard to understand but after a while you will slowly learn through the example and for any question just leave me a comment =)